In recent years, electronic apparatus represented by Information Technology (IT) have been widely popularized due to development of digital technology, and energy consumption caused by the popularization becomes controversial. In the above information technology, communication technology between mobile devices by communication has also progressed.
Generally, a power supply circuit system of a series regulator using a power transformer is used in simple stationary electronic devices. In the power supply system, 100V of commercial power is divided to be fallen and rectified in a diode bridge circuit, then, a waveform is smoothed by a large-capacity capacitor and voltage variation is absorbed and voltage is stabilized by a regulator which is a semiconductor component in order to stabilize variation of external AC voltage, variation of electronic components and output variation.
However, absorption of voltage variation by the regulator has an adverse effect on environmental concerns because the variation amount is discharged as excess Joule heat. The present inventors have proposed a method of reducing discharge of excess Joule heat by using a variable capacitor as described later.
FIG. 17 shows characteristics of a ceramic capacitor in related art, namely, DC bias voltage dependence of capacitance. The ceramic capacitor is typified by two types when broadly divided, which are a B-characteristic capacitor having small capacitance variation with respect to the DC bias voltage as control voltage and an F-characteristic capacitor having large capacitance variation with respect to the DC bias voltage. The F-characteristic shown in FIG. 17 is a use characteristic having capacitance variation of +30 to −80% in the use temperature range of −25° C. to 85° C. The B-characteristic is a use characteristic having capacitance variation of ±10% in the use temperature range of −25° C. to 85° C. As it is desirable that capacitance does not vary as a commonly used capacitor, the B-characteristic capacitor having small capacitance variation is used.
The above capacitors are made of ferroelectric materials of a type of barium titanate (BaTiO3) in which capacitance variation occurs by the DC bias voltage. It is considered that the phenomenon in which capacitance varies by the external voltage occurs by polarization charge caused by capacitor capacitance and the behavior of a polarization domain acting on an electric field. The capacitance variation can be increased by increasing the capacitance by changing the Curie point or by changing the grain size according to additive elements and sintering conditions (see Non-Patent Document 1) in this type of materials. However, it is not preferable because the capacitance varies by adding voltage in general use of the capacitor.
To use the type of barium titanate and lead zirconate titanate (PZT) as the ferroelectric materials for the capacitor is disclosed in Patent Documents 1, 2 and the like.
Patent Document 1: JP-A-10-223475
Patent Document 2: JP-A-2000-101345
Non-Patent Document 1: Landolt-Bornstein Vol. 16, Ferroelectrics and Related Substances (1981)